In effect, the design of electrical aeroplanes is becoming a reality with control surfaces which are actuated by electrical cylinder actuators, and likewise, for Ariane 6, the hydraulic actuators which, on Ariane 5, control the trajectory of the launch vehicle by steering the divergent nozzles of the solid boosters and of the motor of the cryogenic main stage will be replaced by electrical actuators of very high power.
Access to such power levels can be obtained only by stepping up the power supply voltage. For example, for the top stage of Ariane 5, the electrical power necessary for each of the two axes is of the order of 5 kW (at 150 V) whereas, for the solid stages of Ariane 6, the electrical power necessary for each axis will be of the order of 70 kW (at 350V).
Elsewhere, new aerospace applications, such as the Stratobus, project, an autonomous dirigible flying just above air traffic, at 20 kilometres of altitude, can culminate only in an “all-electrical” context.
The criticality of the missions that are now devolved to the electrical actuators demands the development of solutions which, while being failure-tolerant, do not disturb the mission upon the occurrence of a failure.
Also, the competition exerted in the aerospace markets demands the implementation of ever-more economically competitive solutions.
At the core of an electrical actuator there is an electronic control unit (ECPU: Electronic Control and Power Unit); the part which drives the electric motor is composed of a power inverter whose various failure modes are not allowed to affect the mission.
The power electronics in the high-reliability applications like aerospace need architectures capable of fulfilling the mission in the event of a simple failure.
In other words, it is necessary to provide a competitive solution which guarantees a continuity of service in the event of failure of an electronic component, without reconfiguring and without degrading the performance levels of the inverter.
Two families of inverters are in particular known for producing a failure-tolerant inverter.
Reconfigurable inverters are known, in which fuses are added in series with the power switches, even power switches in the phase lines to force the blowing of the fuses in certain failure cases, even power switches for isolating the failing phase and switch it to a standby inverter arm.
This type of architecture presupposes that a simple failure has not caused failure propagation to the other components of the inverter, even short-circuiting of the battery, that all the potential failures can be identified unambiguously, that the fuses blow in all the cases and that the reconfiguration switches do not require a more complex implementation than the redundancy of the inverter arms.
Furthermore, the operation of these inverters assumes the availability of failure detection means and of conducting the appropriate action of reconfiguration of the inverter which temporarily inhibits the operation thereof.
Thus, the applications which demand total availability of the inverter will not be able to use this type of architecture because, upon the occurrence of a simple failure, the operation of the inverter is degraded, even stopped, and this lack of availability lasts for the time required to detect the failure, to identify the corrective action to be carried out and to conduct the corrective action. In the case of Ariane 6, the mission of a solid stage lasts approximately 2 minutes and 20 seconds during which the launch vehicle leaves the launch pad to reach an altitude of 70 kilometres. Such a mission demands permanent availability of the equipment.
One known means for managing the risks of failure consists in designing inverters with redundancy based on the multiplication of the active components for the failure of one component to be compensated, naturally, by a redundant component. To safeguard against the various failure cases, this type of architecture leads to quadrupling the components:                to avoid the effect of the short-circuit failure of a component, a second component is added in series; if it is a controlled component, the control is also duplicated.        To avoid the effect of an open circuit failure of a component, a second component is added in parallel; if it is a controlled component, the control is also duplicated.        
Similarly, in the switched-mode power converters embedded in the satellites, the architectures of the switching cells have to be able to naturally compensate the failure of a component.
The major defect with these solutions is the multiplication of the components which increases the costs and, by increasing the size of the implementation, by also increasing the stray inductances which generate switching overvoltages.
One known means for increasing the current-carrying capacity of a power device is to use the parallel-connection of subsets of lesser capacity, such that their sum capacity equals the required capacity; this condition is valid only if the distribution of the current between the difference subsets is equal.